Osteoporosis is a leading cause of morbidity and mortality, and a serious public health hazard in the United States and throughout the world. To prevent and treat osteoporosis effectively, various predisposing factors (genetic, environmental, endocrine) must be understood. It has been shown that black subjects have a greater skeletal mass and are less prone to vertebral and hip fractures than white subjects, but the reasons for these differences are not clear. The main goal of this study is to explore the basic differences in bone mass, structure and remodeling between blacks and whites, and the responses of the skeleton and kidney to hormonal influences which control bone turnover, particularly the PTH-vit D endocrine system. Better appreciation of these differences will lead to a better understanding of the pathogenesis of osteoporosis. We will examine these differences in a well defined population. The specific aims will be as follows: 1) To evaluate bone structure in blacks and whites, particularly the structural organization of cancellous bone and thickness of cortical bone, using histomorphometry in iliac bone biopsies. 2) To determine if the rate of bone turnover is different in blacks than in whites, using dynamic variables of bone histomorphometry after in vivo tetracycline labelling as well as biochemical indices of bone turnover. 3) To compare the renal response to stimulation by (1-34) hPTH in black and white subjects, and determine if the 1-alpha hydroxylase response and consequently 1,25(OH)2D production is different in blacks. 4) To compare the sensitivity of the skeleton of blacks and whites to stimulation by PTH and 1,25(OH)2D. 5) To determine if the responsivity of the parathyroid gland to a hypocalcemic stimulus is different in blacks and whites.